34億年前の海洋に生物的硫黄代謝の痕跡 ―太古の浅瀬は生命にとっての“硫黄のオアシス”だった？―

2026-04-16 東京大学,千葉大学,東北大学,名古屋大学

オーストラリア、約34億年前の堆積岩とその中に見つかる微小な同心円状の黄鉄鉱

＜関連情報＞

• https://www.aori.u-tokyo.ac.jp/research/news/2026/20260416.html
• https://www.sciencedirect.com/science/article/pii/S0016703726001419

34億年前のコロフォーム黄鉄鉱に保存された微生物活動：マイクロスケールの硫黄同位体分析 Microbial activity preserved in 3.4 Ga colloform pyrite: A micro–scale sulfur isotope analyses

Kohei Sasaki, Naoto Takahata, Akizumi Ishida, Takeshi Kakegawa, Kenichiro Sugitani
Geochimica et Cosmochimica Acta  Available online: 7 March 2026
DOI:https://doi.org/10.1016/j.gca.2026.03.005

Abstract

Microfossil-bearing black cherts from the ca. 3.4 Ga Strelley Pool Formation in the Goldsworthy Greenstone Belt, Pilbara Craton, offer critical insights into early Archean microbial ecosystems. In this study, we analyzed concentric colloform pyrite grains in these cherts to investigate their formation mechanisms and potential microbial involvement. The examined chert samples contain abundant organic matter and microfossils. The colloform pyrite grains, typically less than 10 µm in diameter, exhibit high sphericity and concentric internal growth structures. Their spatial association with diagenetic silica veins suggests pyrite formation prior to lithification. In situ sulfur isotope analyses using nanoscale secondary ion mass spectrometry (NanoSIMS) revealed large intra-grain δ³⁴S variations, up to 28.6‰. δ³⁴S values reach as low as –20.1‰ (VCDT) in the innermost cores and progressively increase toward the outer layers, where they average approximately +4‰. These trends are consistent with Rayleigh-type isotope fractionation under sulfate-limited conditions, likely mediated by heterotrophic microbial sulfate reduction. Secondary ion mapping of ¹²C⁻ and ¹²C¹⁴N⁻ revealed alternating layers of organic matter and pyrite. The close spatial association between pyrite and organic matter with significant ¹²C-enriched carbon isotope signatures, further supports a biogenic origin. Collectively, these findings provide new microscale geochemical evidence for microbial sulfur metabolism in early Archean sedimentary environments.